Beds of solid mineral particles are often transported during heating or cooling on a horizontal traveling grate comprising parallel grate plates, or grate bars affixed at their ends to endless chains which engage sprocket drive wheels. For example, horizontal traveling grates are used to convey beds of solid mineral particles in limestone, cement and iron ore indurating systems while heating or cooling gas streams are forced through the bed carried on the grate plates. A significant amount of heat energy is wasted as the grate plates and grate chain return. The bed of mineral particles is esentially static as it is transported by the traveling grate, and a relatively high drop exists in the pressure across the bed required to blow the heat tranfer gas stream through the bed. Further, relatively high horsepower motors are required to drive the traveling grate chain and the fans which force the gas streams through the bed.
Screens for conveying and sizing material particles such as green taconite pellets are also known which comprise a series of driven, spaced-apart, parallel, screen rods, or rollers, such as disclosed in U.S. Pat. Nos. 2,988,781; 3,438,491 and 3,848,744. The particles are transported in a single layer at a speed approaching the circumferential velocity of the rollers, the rollers may be of different diameter and driven at different speeds to convey the pellets, the spacing between rollers is varied to classify the pellets according to size, and the rollers may be covered with abrasive resistant rubber, as taught in U.S. Pat. No. 3,438,491, or may have a hard chromium outer shell, as taught in U.S. Pat. No. 3,848,744, to extend the service life of the rollers. Such conveying and sizing roller screens are not adapted to exchange heat with the pellets and are incapable of transporting a deep bed of pellets at slow velocity while forcing a heat transfer gas stream through the bed.